Interstitial radiation therapy using 125I seeds was initiated at Memorial Hospital in 1965 and is now being performed at more than 90 institutions in the United States. Its therapeutic utility has been well established. The low energy photons (average energy 28 keV) emitted by 125I, however, have made difficult the physical dosimetry upon which the clinical assessment of radiation dose must be based. State-of-the-art measurements made in conjunction with the early clinical use of 125I seeds have been re-evaluated and the need is apparent for new measurements using improved methods and instrumentation which have become available in the interim. The measurements described in the present proposal are expected to provide the required accuracy in the reference data base for routine computer calculations of absorbed dose distribution in patients. A hemispherical ionization chamber of conducting plastic will be used to measure the average dose rate at about 1 cm from an 125I seed which will then be used as a standard for routine calibrations. The chamber wall material be tissue-equivalent, or correctable to tissue equivalent, with respect to the kerma from 125I photons. Exchangeable central electrodes will permit varying gap width for extrapolation to zero width. Relative dose distribution measurements in a plane containing the source will be made using LiF thermoluminescent dosimeters in a solid phantom material specially selected for tissue equivalence with respect to spatial attentuation of 125I photons. These data will be normalized to the hemispherical chamber measurement. They will be compared with calculated dose distribution data based on the standard source activity inferred from a measurement of exposure. The exposure measurement will be accomplished with a thin-walled ionization chamber of a type recently developed for mammography, having flat response characteristics in the range 7 to 35 keV. Angular distribution measurements of exposure will back up the LiF dosimetry data in the construction of a two-dimensional table of single source dose distribution for use in computer calculations to evaluate the effect of asymmetry on idealized implants.